A 32-year-old man from rural India presents with recurrent pyogenic infections (Staphylococcus aureus, Streptococcus pneumoniae) and a history of meningococcal meningitis. Serum complement levels are normal, but he has absent serum bactericidal activity. Which complement component deficiency is most commonly associated with recurrent meningococcal infection?

Explanation

## Complement Component Deficiencies and Meningococcal Infection ### Clinical Presentation Analysis The patient has: - **Recurrent meningococcal meningitis** (hallmark presentation) - Normal total complement levels (rules out severe global deficiency) - **Absent serum bactericidal activity** (inability to kill bacteria in serum) - Recurrent pyogenic infections (S. aureus, S. pneumoniae) This constellation points to a **terminal complement pathway deficiency**. ### Complement Component Deficiencies and Clinical Associations | Component | Deficiency Type | Most Common Infection | Mechanism | Frequency | |-----------|-----------------|----------------------|-----------|----------| | **C5** | Terminal pathway | **Neisseria meningitidis** (meningitis) | Loss of C5a (chemotaxis) + MAC formation | **Most common** | | C6, C7, C8, C9 | Terminal pathway | N. meningitidis | Impaired MAC assembly | Less common | | C3 | Central hub | Encapsulated bacteria (S. pneumoniae, H. influenzae) | Loss of opsonization + MAC | Severe, early-onset | | C1q, C2, C4 | Classical pathway | SLE-like disease, encapsulated bacteria | Impaired opsonization | Autoimmune manifestations | ### Why C5 Deficiency? **Key Point:** C5 deficiency is the **most common complement deficiency associated with meningococcal disease** in the literature. **High-Yield:** C5 is critical for two functions: 1. **C5a generation**: Potent chemotactic fragment that recruits neutrophils to infection site 2. **MAC assembly initiation**: C5b is the first component of the membrane attack complex (C5b–C9) **Without C5:** - Impaired neutrophil recruitment → reduced local inflammatory response - No MAC formation → bacteria (especially Neisseria meningitidis) survive in serum - **Serum bactericidal activity is absent** (patient cannot kill bacteria in vitro) - Neisseria meningitidis is **uniquely vulnerable** to MAC; other bacteria are more resistant **Clinical Pearl:** Meningococcal meningitis in an adult with normal total complement levels should raise suspicion for **late complement component deficiency (C5–C9)**. This is a classic board presentation. ### Why Not the Other Options? ```mermaid flowchart TD A[Recurrent Meningococcal Infection]:::outcome --> B{Complement Level?}:::decision B -->|Normal| C[Terminal Pathway Defect]:::action B -->|Low| D[Early Pathway or C3 Defect]:::action C --> E[C5-C9 Deficiency]:::outcome E --> F[Absent MAC + Impaired Chemotaxis]:::action D --> G[C1q/C2/C4 or C3 Deficiency]:::outcome G --> H[SLE-like or Encapsulated Bacteria Risk]:::action ``` **C1q deficiency:** - Causes SLE-like disease, not meningococcal infection - Presents with autoimmune manifestations (rash, arthritis, glomerulonephritis) - Does not explain absent bactericidal activity **C3 deficiency:** - Causes recurrent infections with encapsulated bacteria (S. pneumoniae, H. influenzae) - Would show **low serum C3 levels** (not normal as in this case) - Less specific for meningococcal disease **C4 deficiency:** - Part of classical pathway; rare cause of meningococcal infection - Associated with SLE-like disease - Normal complement levels would not fit **Mnemonic: "LMNOP" for Late Complement Deficiency:** - **L**ate pathway (C5–C9) - **M**eningococcal infection (Neisseria meningitidis) - **N**ormal total complement - **O**psonization intact (pyogenic bacteria still killed by antibodies + C3b) - **P**athognomonic for meningococcal disease [cite:Harrison 21e Ch 372] ## Summary **C5 deficiency is the most common complement component deficiency presenting with recurrent meningococcal meningitis** despite normal total complement levels. The absent serum bactericidal activity reflects loss of MAC-mediated killing.